diff --git a/test/__fixtures__/ignore-unknown/.prettierignore b/test/__fixtures__/ignore-unknown/.prettierignore
new file mode 100644
index 0000000..1615b0c
--- /dev/null
+++ b/test/__fixtures__/ignore-unknown/.prettierignore
@@ -0,0 +1 @@
+ignored.js
diff --git a/test/__fixtures__/ignore-unknown/.prettierrc b/test/__fixtures__/ignore-unknown/.prettierrc
new file mode 100644
index 0000000..df113ef
--- /dev/null
+++ b/test/__fixtures__/ignore-unknown/.prettierrc
@@ -0,0 +1,10 @@
+{
+  "overrides": [
+    {
+      "files": "*.as-js-file",
+      "options": {
+        "parser": "babel"
+      }
+    }
+  ]
+}
diff --git a/test/__fixtures__/ignore-unknown/ignored.js b/test/__fixtures__/ignore-unknown/ignored.js
new file mode 100644
index 0000000..e69de29
diff --git a/test/__fixtures__/ignore-unknown/javascript.js b/test/__fixtures__/ignore-unknown/javascript.js
new file mode 100644
index 0000000..b029879
--- /dev/null
+++ b/test/__fixtures__/ignore-unknown/javascript.js
@@ -0,0 +1 @@
+const foo=        "bar";
diff --git a/test/__fixtures__/ignore-unknown/override.as-js-file b/test/__fixtures__/ignore-unknown/override.as-js-file
new file mode 100644
index 0000000..b029879
--- /dev/null
+++ b/test/__fixtures__/ignore-unknown/override.as-js-file
@@ -0,0 +1 @@
+const foo=        "bar";
diff --git a/test/__fixtures__/ignore-unknown/unkown-file-extension.unknown b/test/__fixtures__/ignore-unknown/unkown-file-extension.unknown
new file mode 100644
index 0000000..e2358dc
--- /dev/null
+++ b/test/__fixtures__/ignore-unknown/unkown-file-extension.unknown
@@ -0,0 +1 @@
+PRETTIER
diff --git a/test/__fixtures__/ignore-unknown/unkown-filename b/test/__fixtures__/ignore-unknown/unkown-filename
new file mode 100644
index 0000000..e2358dc
--- /dev/null
+++ b/test/__fixtures__/ignore-unknown/unkown-filename
@@ -0,0 +1 @@
+PRETTIER
diff --git a/test/__tests__/__snapshots__/ignore-unknown.js.snap b/test/__tests__/__snapshots__/ignore-unknown.js.snap
new file mode 100644
index 0000000..6e1e56e
--- /dev/null
+++ b/test/__tests__/__snapshots__/ignore-unknown.js.snap
@@ -0,0 +1,44 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`Ignored file (stderr) 1`] = `""`;
+
+exports[`Ignored file (stdout) 1`] = `""`;
+
+exports[`Ignored file (write) 1`] = `[]`;
+
+exports[`None exist file (stderr) 1`] = `"[error] No files matching the given patterns were found"`;
+
+exports[`None exist file (stdout) 1`] = `""`;
+
+exports[`None exist file (write) 1`] = `[]`;
+
+exports[`Not matching pattern (stderr) 1`] = `"[error] No files matching the given patterns were found"`;
+
+exports[`Not matching pattern (stdout) 1`] = `""`;
+
+exports[`Not matching pattern (write) 1`] = `[]`;
+
+exports[`ignore-unknown alias (stdout) 1`] = `
+"javascript.js
+override.as-js-file"
+`;
+
+exports[`ignore-unknown check (stderr) 1`] = `
+"[warn] javascript.js
+[warn] override.as-js-file
+[warn] Code style issues found in 2 files. Run Prettier with --write to fix."
+`;
+
+exports[`ignore-unknown check (stdout) 1`] = `"Checking formatting..."`;
+
+exports[`ignore-unknown check (write) 1`] = `[]`;
+
+exports[`ignore-unknown dir (stdout) 1`] = `
+"javascript.js
+override.as-js-file"
+`;
+
+exports[`ignore-unknown pattern (stdout) 1`] = `
+"javascript.js
+override.as-js-file"
+`;
diff --git a/test/__tests__/ignore-unknown.js b/test/__tests__/ignore-unknown.js
new file mode 100644
index 0000000..d28d602
--- /dev/null
+++ b/test/__tests__/ignore-unknown.js
@@ -0,0 +1,75 @@
+import { runCli } from "../utils";
+
+describe("ignore-unknown dir", () => {
+  runCli("ignore-unknown", [
+    ".",
+    "--ignore-unknown",
+    "--list-different",
+  ]).test({
+    status: "non-zero",
+    stderr: "",
+    write: [],
+  });
+});
+
+describe("ignore-unknown alias", () => {
+  runCli("ignore-unknown", [
+    ".",
+    "-u",
+    "--list-different",
+  ]).test({
+    status: "non-zero",
+    stderr: "",
+    write: [],
+  });
+});
+
+describe("ignore-unknown pattern", () => {
+  runCli("ignore-unknown", [
+    "*",
+    "--ignore-unknown",
+    "--list-different",
+  ]).test({
+    status: "non-zero",
+    stderr: "",
+    write: [],
+  });
+});
+
+
+describe("ignore-unknown check", () => {
+  runCli("ignore-unknown", [
+    ".",
+    "--ignore-unknown",
+    "--check",
+  ]).test({
+    status: 1,
+  });
+});
+
+describe("None exist file", () => {
+  runCli("ignore-unknown", [
+    "non-exist-file",
+    "--ignore-unknown",
+  ]).test({
+    status: 1,
+  });
+});
+
+describe("Not matching pattern", () => {
+  runCli("ignore-unknown", [
+    "*.non-exist-pattern",
+    "--ignore-unknown",
+  ]).test({
+    status: 1,
+  });
+});
+
+describe("Ignored file", () => {
+  runCli("ignore-unknown", [
+    "ignored.js",
+    "--ignore-unknown",
+  ]).test({
+    status: 0,
+  });
+});
diff --git a/test/utils.js b/test/utils.js
index bb87d7f..604d68d 100644
--- a/test/utils.js
+++ b/test/utils.js
@@ -94,10 +94,10 @@ async function runCommand(dir, args, options) {
   return { status, stdout, stderr, write };
 }
 
-async function runTest(name, expected, result, options) {
+async function runTest(name, expected, lazyResult, options) {
   const title = options.title || "";
   test(`${title}(${name})`, async () => {
-    const value = (await result)[name];
+    const value = (await lazyResult())[name];
     if (expected !== undefined) {
       if (name === "status" && expected === "non-zero") {
         expect(value).not.toBe(0);
@@ -113,24 +113,31 @@ async function runTest(name, expected, result, options) {
 }
 
 function runCli(dir, args = [], options = {}) {
-  const promise = runCommand(dir, args, options);
+  let promise;
+  const lazyPromise = async () => {
+    if (promise) {
+      return promise;
+    }
+    promise = runCommand(dir, args, options);
+    return promise;
+  };
   const result = {
     get status() {
-      return promise.then((result) => result.status);
+      return lazyPromise().then((result) => result.status);
     },
     get stdout() {
-      return promise.then((result) => result.stdout);
+      return lazyPromise().then((result) => result.stdout);
     },
     get stderr() {
-      return promise.then((result) => result.stderr);
+      return lazyPromise().then((result) => result.stderr);
     },
     get write() {
-      return promise.then((result) => result.write);
+      return lazyPromise().then((result) => result.write);
     },
     test: (tests) => {
       for (const name of ["status", "stdout", "stderr", "write"]) {
         const expected = tests[name];
-        runTest(name, expected, promise, options);
+        runTest(name, expected, lazyPromise, options);
       }
       return result;
     },